In this paper, an “interfacial boundary volume” based damage criterion was proposed in combination with the modified Coffin-Manson model to predict solder fatigue. This criterion assumes that mainly, the behavior of the thin solder layer at chip pad interface contributes to the solder fatigue, not the whole solder joint or the averaged strains from randomly selected elements. The damage parameter was thus calculated by averaging the visco-plastic strain range over the interfacial boundary layer volume in the solder and later related to the corresponding fatigue life of experimental test through least-squares curves fitting to determine the empirical coefficients in the Coffin-Manson equation. As a demonstrator, the solder joint fatigue in wafer level chip scale packaging under thermal shock loading was analyzed. An appropriate constitutive relation from Darveaux was used to model the inelastic deformation of the solder alloy, and the different stress-strain responses resulting from different designs were calculated. The analysis results were used to develop the empirical fatigue model based on the interfacial boundary volume damage criterion and then this fatigue model was used for prediction. The fatigue lives of chip scale packaging with variable solder land size and component size were analyzed using this model. The prediction results match well with those from experimental tests. For this demonstrator, it was also shown that the empirical model based on the interfacial boundary volume criterion was more accurate than the models obtained from other strain averaging methods.
